Surge arrestor utilizing quarter wave stubs



Nov. 29, 1966 R. F. KEARNS ETAL 3,239,117

SURGE ARRESTOH UTILIZING QUARTER WAVE STUBS 2 Sheets-Sheet 1 Filed March 23, 1964 INVENTORS. ROBERT F. KEARNS and FRANCIS H. STITES ATTORNEY.

Nov. 29, 1966 R. F. KEARNS ETAL 3,28

SURGE ARRESTOR UTILIZING QUARTER WAVE STUBS 2 Sheets-Sheet 2 Filed March 25, 1964 O O O O 2 3 4 5 K JO 200 F REQUENCY (M C) [FIG.4

FREQUENCY (MC) INVENTORS. ROBERT F. KEARNS and FRANCIS H. STITES ATTORNEY.

3,239,117 Patented Nov. 29, 1966 files 3,289,117 SURGE ARRESTOR UTILIZING QUARTER WAVE STUBS Robert F. Kearns, Westwood, and Francis H. Stites, Wayland, Mass., assignors to Sylvania Electric Products Inc.,

a corporation of Delaware Filed Mar. 23, 1964, Ser. No. 353,721 7 Claims. (Cl. 333-73) This invention relates to surge arrestors and more particularly to broadband electronic surge arrestors having no low frequency coupling between input and output terminals.

To protect electronic equipment from damage caused by excessive voltage and current such as caused by lightning, nuclear explosion, or other electromagnetic disturbance, various surge arrestors have been devised which by-pass these voltage and current surges to ground. To protect communications equipment, for example, surge arrestors are generally disposed between the antenna and the transmitter and receiver and are designed to divert surges to ground rather than allowing them to enter the equipment. In order to provide adequate equipment protection, the arrestor must by-pass the entire surge to ground and not allow any surge voltage or current to reach the equipment. In addition, the surge arrestor should not impair the electrical operation of the equip ment. In general, spark gaps or protective diodes have heretofore been employed to protect such equipment in the presence of a surge. A major disadvantage of spark gaps, however, is that they are themselves susceptible to damage by voltage and current surges and must, therefore, be replaced quite often in order to maintain the requisite level of protection. Another serious disadvantage is that the shunt capacitance of such devices is relatively high which limits the bandwidth of the communications apparatus being protected. This is especially deleterious in microwave communication systems which, typically, can have bandwidths of the order of 200 megacycles. Improved, highly reliable spark gap arrestors have been developed for some applications; however, their use in broadband, high frequency systems is prevented by their inherent high capacitance. Spar-k gaps, therefore, have limited usefulness in protecting microwave equipment since the limited bandwidth of these gaps renders them incapable of operation over the bandwidths encountered in such equipment. Protective diode circuits, in which suitably biased diodes break down in the presence of excess voltage, also suffer the disadvantage of high capacitance at microwave frequencies.

With an appreciation of the foregoing disadvantages of present surge arrestors, it is a primary object of the present invention to provide a broadband, highly reliable surge arrestor especially useful in protecting microwave apparatus.

Another object of the invention is to provide a surge arrestor having no low frequency coupling between input and output terminals.

Another object of the invention is to provide a surge arrestor having an input circuit directly grounded but which is decoupled from the output circuit at specified frequencies.

It has been found that the frequency spectrum of electromagnetic disturbances caused, for example, by lightning or nuclear exposition, is below 100 megacycles. In order to protect electronic equipment operating in the frequency spectrum above 100 megacycles it has been discovered that a bandpass filter of novel design can be employed to provide the requisite protection without adversely affecting the electrical operation of the equipment. Briefly, the invention comprises a resonant line bandpass filter that transmits energy in the desired operating frequency band, while energy in a lower frequency band due to lightning or other electromagnetic disturbance is conducted directly to ground. Since there is no coupling through the device in the lower frequency band, surge currents pass directly to an external ground in the input circuit of the arrestor. The equipment ground, therefore, need not be constructed to carry heavy surge currents since these currents never reach the equipment ground.

The foregoing, together with other objects features, and advantages of the present invention will be more apparent from the following detailed description, taken in conjunction with the accompanying drawing in which:

FIG. 1 is a schematic representation of the surge arrestor of the invention;

FIG. 2 is a pictorial view of a preferred embodiment of the invention;

FIG. 3 is a cross-sectional view taken along line 33 of FIG. 2;

FIG. 4 is a plot of the measured bandwidth of the surge arrestor of FIGS. 2 and 3; and

FIG. 5 is a plot of the measured standing wave ratio of the device.

The surge arrestor of the invention is essentially a transmission line bandpass filter, a schematic representation of which is shown in FIG. 1. It consists of an input section 10, an output section 20, a pair of short circuited quarter wave stubs 12 and 14, respectively connected across the input and output sections, and a pair of open circuited quarter wave stubs 16 and 18 coupling the input and output circuits of the filter. The line lengths of sections 10 and 20 are not critical as the requisite filter operation is determined essentially by the dimensions of the open and shorted quarter wave stubs.

It will be noted that a direct path to ground is provided at the input and output filter via shorted stubs 12 and 14, respectively. It will also be noted that low frequency isolation is provided between the input and output by means of open stubs 16 and 18. In this manner, low frequency surges are channeled directly to ground in the input circuit of the filter, and are not coupled to the output circuit where they could be transmitted tothe equipment. On the other hand, a signal applied to input section 10 which has a frequency within the pass band of the filter is transmitted unimpaired to output section 20, and thence to subsequent circuitry.

The filter is designed to have a pass band in the fre quency range of the communications equipment being protected. This pass hand must, of course, be separated from the frequency band of the electromagnetic disturbance by a suitable amount so that filtering can be accomplished. The frequency spectrum of electromagnetic disturbances caused, for example, by lightning or nuclear explosion, is below 100 megacycles, with the major spectrum density well below 100 me-gacycles; thus, the present invention is usable at any frequency range above this 100 megacycle threshold. The operating frequency range of a particular filter, and the location of a pass band having a specified range, are calculable by well known filter design methods. The line lengths of the open and short circuited stubs are, of course, chosen to be one quarer w-ave long at the center frequency of the operating pass band in question.

In practice, the invention can be embodied in many well known transmission line forms. It lends itself particularly to coaxial construction as illustrated in the embodiment of FIG. 2 wherein the entire device is contained Within first and second conductive colinear outer cylinders 30 and which are connected together by means of respective block halves 48a and 48b. As can best be seen from FIG. 3, a first inner cylinder 32 having a beveled inner end is coaxially supported within outer cylinder 30 and conductively connected at its outermost end to outer cylinder 30 via a conductive plate 34. A second inner cylinder 36 also having a beveled inner end is coaxially supported within outer cylinder 80 colinear with cylinder 32 with the beveled ends confronting each other. Cylinders 32 and 36 are beveled as shown to maintain symmetry and thereby reduce impedance mismatch. A conductive rod 38 is disposed coaxially within cylinders 32 and 36 and electrically connected at its right end to cylinder 36 by means of a conductive plate 40. As can be seen most clearly in FIG. 2, outer cylinders 30 and 80 are conductively secured to a metal block 48 which is constructed in two halves 48a and 48b to facilitate assembly and disassembly of the filter. Thus, cylinder 30 is secured to block half 48a, while cylinder 80 is attached to block half 4812. The two halves of block 48 are fastened together by machine screws 88 or other suitable means. Typically, the entire conductive structure is made of copper or brass, the electrical connections between elements being provided, for example, by brazing or silver soldering.

Referring again to FIG. 3, the inner conductor 42 of coaxial input terminal 44 passes through a hole in block 48 and is connected to the inner end of cylinder 32, while the outer conductor 46 of input terminal 44 is connected to outer cylinders 30 and 84 via conductive block 48. The inner conductor of coaxial output terminal 52 is connected to conductive rod 38 at approximately its midpoint via conductor 50 which passes through a suitably placed hole in the end of cylinder 36, while the outer conductor is connected to the inner end of cylinder 36 via cylinder 54. The length of the output line is not critical but can be dimensioned to satisfy particular installation requirements. Insulating spacers, which are typically made of Teflon, are provided to maintain the spacing between conductive rod 38 and cylinders 32, 36, 30 and 80. Insulating spacers 56, 58 and 60 center rod 38 within cylinders 32 and 36. Similarly, cylinder 32 is positioned within cylinder 30 by spacer 62, while spacers 64 and 66 locate cylinder 36 within cylinder 80. A dielectric spacer 70 is provided in the lower face of block 48 to insulate and mechanically support cylindrical conductor 54 with respect to block 48, and a dielectric spacer 90 is provided as shown to insulate conductor 50 from cylinder 36. Radio frequency shielding is provided by a metal plate 68 attached to the outer end of cylinder 80. As is well known in the microwave art, conductive rod 38 and cylinders 32 and 36 are undercut slightly at the positions of the corresponding dielectric spacers to compensate for the impedance mismatch due to the presence of the dielectric material. This undercutting also serves to securely seat the spacers at the desired locations.

It will be apparent that inner cylinder 32 and outer cylinder 30 comprise the short circuited stub 12 across the input circuit in FIG. 1 and that the inner cylinder 36 and the right half of conductive rod 38 form the short circuited stub 14 across the output circuit 20. The open circuited stub connecting the center conductors of the input and output circuits consists of the cylinder 32 and the left half of conductive rod 38, while the open circuited stub connecting the outer conductors consists of outer cylinder 80 and the inner cylinder 36. It will be recognized that these open circuited stubs correspond to stubs 16 and 18 of FIG. 1. The open and short circuited :sections are substantially one quarter wavelength long .at the center frequency of operation, slight departure from an exact quarter wavelength being due to detun- 'ing caused by capacitive end effects. For example, capacitive end effects are present between the open circuited end of rod 38 and cylinders 30 and 32, between the short circuited end of cylinder 36 and cylinder 80, and between the beveled edges of cylinders 32 and 36. Tuning to the particular frequency band of interest, as well as adjusting the impedance and standing wave ratio, is accomplished in the well known manner by suitably adjusting the lengths of the several stub sections to achieve the desired result.

As shown in FIG. 2, mounting flanges 84 and 36, connected respectively to end plates 34 and 68, are provided to fasten the surge arrestor to a grounded structure, for example an antenna tower. The surge arrestor can be additionally fastened to the grounded structure by bolts (not shown) threaded into the lower face of block 48 to provide further electrical connection to ground. In the usual installation, an antenna cable is connected to the input terminal 44 by a suitable connector, and the output terminal 52 is connected by a coaxial cable to the transmitter/receiver of the communication equipment. In operation, a lightning surge, or other electromagnetic disturbance, inducml in the antenna will be conducted by the antenna cable to the surge arrestor whence it will be directed to ground through the outer body of the arrestor in the manner explained hereinbefore. Since the spectrum of the disturbance is essentially below megacycles, which is below the pass band of the filter, the surge cannot be coupled to the output circuit, but rather, is conducted directly to ground via the short circuited stub in the input circuit of the arrestor. It is evident that the equipment ground need not be designed to carry heavy surge currents as these surge currents are diverted to ground in the input circuit of the arrestor and, therefore, never reach the equipment ground. A received signal within the pass band of the filter, on the other hand, is coupled through the device to the receiver with little attenuation or distortion. The arrestor being reciprocal, a signal from the transmitter is similarly coupled through the filter to the antenna.

In an embodiment that was constructed and designed to operate in a 225400 megacycle communication system, a surge arrestor having a characteristic impedance of 50 ohms was constructed of copper tubing, the outer cylinder having a wall thickness of inch, and the inner cylinder having a inch wall thickness. The overall length of the arrestor is approximately 20 inches, with an outside diameter of approximately 2.5 inches. The extremely broad bandwidth of the filter is illustrated by the plot of attenuation versus frequency in FIG. 4. From this curve, it can be seen that a three db bandwidth of approximately 300 megacycles is available, which is more than adequate for most microwave systems. The additional higher frequency pass band evident in the curve, which is common to coaxial structures, is caused by additional coupling due to the tuned stubs being multiples of a quarter wavelength at higher frequencies, but does not impair the operation of the filter in its designed operative region. The measured standing wave ratio is depicted in FIG. 5, wherein the solid curve shows the standing wave ratio before a surge was applied to the filter, while the dotted curve shows the standing wave ratio after the filter has conducted several surges. The standing wave ratio in both instances is generally below 1.2 and it is apparent that repeated applied surges has little effect on the performance of the filter. Surge voltages of 50,000 volts were applied to the filter without adverse effect, and currents of 4,000 amperes were conducted with no ill effect. It is expected that greater voltages and currents could be handled than those stated above; the factors limiting the magnitude of the surges being the coaxial connectors and connecting cables, rather than the filter itself. Similarly, the maximum operating signal level is limited, not by the filter, but, rather, by the size and characteristics of the connectors. Connectors are, however, commercially available to withstand signal powers normally encountered in practice.

Although the invention is especially useful in an electronic surge arrestor, it can also be used as a bandpass filter in many well known microwave applications. The filter can, for example, function as a muticoupler to separately coupled signals from two antennas to their respective receivers.

While there has been shown what is now thought to be a preferred embodiment of the present invention, many modifications and alternative constructions will occur to those skilled in the art without departing from the true spirit and scope of the invention. Accordingly, it is not intended to limit the invention by what has been particularly shown and described, except as defined in the appended claims.

What is claimed is:

1. An electronic surge arrestor comprising; an outer conductive cylinder substantially one half wavelength long at a center frequency of operation, a first conductive inner cylinder substantially one quarter wavelength long at said center frequency, disposed coaxially within one half of said outer cylinder and conductively connected at its outer end to a corresponding end of said outer cylinder, a second conductive inner cylinder substantially one quarter wavelength long at said center frequency disposed coaxially within the other half of said outer cylinder, a conductive rod substantially one half wavelength long at said center frequency coaxially disposed within said first and second conductive cylinders and connected at one end to the corresponding outer end of said second cylinder, a coaxial input terminal having its outer conductor connected to the midpoint of said outer cylinder and its inner conductor connected to the inner end of said first inner cylinder, and a coaxial output terminal having its outer conductor connected to the inner end of said second inner cylinder and its inner conductor connected to the midpoint of said conductive rod.

2. An electronic surge arrestor comprising; an outer conductive cylinder substantially one half Wavelength long at a center frequency of operation, a first inner conductive cylinder substantially one quarter Wavelength long at said center frequency, coaxially disposed within one half of said outer cylinder and connected at its outer end to a corresponding end of said outer cylinder, a second inner conductive cylinder substantially one quarter wavelength long at said center frequency and coaxially disposed within the other half of said outer cylinder, said first and second inner cylinders each having an inner beveled end, a conductive rod substantially one half wavelength long at said center frequency, coaxially disposed within said inner cylinders and connected at one end to the corresponding outer end of said second inner cylinder, a coaxial input terminal having its inner conductor connected to the beveled end of said first inner cylinder and its outer conductor connected substantially midway of said outer cylinder, and a coaxial output terminal having its inner conductor connected to the midpoint of said conductive rod and its outer conductor connected to the beveled end of said second inner cylinder.

3. A surge arrestor operative to isolate electronic equipment from voltage and current surges comprising:

first and second input terminals adapted to be connected to a source of signals; first and second output terminals adapted to be connected to utilization equipment; 5 a short circuited quarter wave stub connected between said first and second input terminals; and

first and second open circuited quarter wave stubs,

said first open circulated quarter wave stub connected between said first input terminal and said first output terminal and said second open circuited quarter wave stub connected between said second input terminal and said second output terminal.

4. The invention, according to claim 3, additionally comprising a second short circuited quarter Wave stub connected between said first and second output terminals.

5. A surge arrestor, according to claim 3, in which said first and second open circuited quarter wave stubs include respective first and second collinearly disposed conductive cylinders each having a confronting beveled inner end.

6. For electronic equipment operating at microwave frequencies, a surge arrestor operative to protect said equipment from voltage and current surges of lower frequencies comprising:

an input circuit for connecting said arrestor to a source of signals;

a short circuited quarter wave stub connected across said input circuit and operative to direct to ground the voltage and current surges of lower frequency;

an output circuit for connecting said arrestor to utilization equipment; and

first and second open circuited quarter wave stubs connected between said input and output circuits to thereby provide signal and ground transmission paths at the microwave frequencies between said input and output circuits.

7. The invention, according to claim 6, additionally comprising a second short circuited quarter wave stub connected across said output circuit.

References Cited by the Examiner- UNITED STATES PATENTS 2,076,248 4/1937 Norton 333-70 2,196,272 4/1940 Peterson 33373 2,532,993 12/1950 Carter 33373 OTHER REFERENCES Very High-Frequency Techniques (Radio Research Laboratory-Harvard University) Reich et 211., McGraw- Hill, New York, 1947, vol. II, pages 669 and 710.

The A.R.R.L. Antenna Book, The American Radio Relay League, Incorporated, West Hartford, Connecticut,

1956, page 79.

HERMAN KARL SAALBACH, Primary Examiner.

M. NUSSBAUM, Assistant Examiner. 

6. FOR ELECTRONIC EQUIPMENT OPERATING AT MICROWAVE FREQUENCIES, A SURGE ARRESTOR OPERATIVE TO PROTECT SIAD EQUIPEMENT FROM VOLTAGE AND CURRENT SURGES OF LOWER FREQUENCIES COMPRISING: AN INPUT CIRCUIT FOR CONNECTING SAID ARRESTOR TO A SOURCE OF SIGNALS; A SHORT CIRCUITED QUARTER WAVE STUB CONNECTED ACROSS SAID INPUT CIRCUIT AND OPERATIVE TO DIRECT TO GROUND THE VOLTAGE AND CURRENT SURGES OF LOWER FREQUENCY; AN OUTPUT CIRCUIT FOR CONNECTING SAID ARRESTOR TO UTILIZATION EQUIPMENT; AND FIRST AND SECOND OPEN CIRCUITED QUARTER WAVE STUBS CONNECTED BETWEEN SAID INPUT AND OUTPUT CIRCUITS TO THEREBY PROVIDE SIGNAL AND GROUND TRANSMISSION PATHS AT THE MICROWAVE FREQUENCIES BETWEEN SAID INPUT AND OUTPUT CIRCUITS. 